1. Field of the Invention
The present invention relates to a flat panel display device, and more particularly, to a digital-analog converter provided in the flat panel display device and a data driver using the digital-analog converter.
2. Discussion of Related Art
A flat panel display device includes a display panel, a scan driver, and a data driver. The scan driver sequentially outputs scan driving signals to a plurality of scan lines formed on the display panel, and the data driver outputs R, G, B image signals to data lines on the display panel. Non-limiting examples of a flat panel display device include a liquid crystal display device, a field emission display device, a plasma display panel, a light emitting display device, etc.
FIG. 1 is a block diagram showing a conventional data driver.
Here, the data driver will be described on the assumption that it has n channels.
Referring to FIG. 1, the data driver includes: a shift register unit 110, a sampling latch unit 120, a holding latch unit 130, a digital-analog converter (DAC) 140, and an amplifier 150.
The shift register unit 110 receives a source shift clock (SSC) and a source start pulse (SSP) from a timing controller (not shown), and generates n sampling signals in sequence, while allowing the source start pulse (SSP) to be shifted for every one period of the source shift clock (SSC). To generate the n sampling signals, the shift register unit 110 includes n shift registers.
The sampling latch unit 120 sequentially stores data in response to the sampling signals supplied from the shift register 110 in sequence. Here, the sampling latch unit 120 is provided with n sampling latches for storing n digital data. Also, the respective sampling latches have sizes corresponding to the number of bits of the data. For example, when the data is configured to have k bits, the respective sampling latches are set to have a size of k bits.
The holding latch unit 130 receives and stores the data from the sampling latch unit 120 when a source output enable (SOE) signal is input. Also, the holding latch unit 130 supplies the data stored therein to a DAC 250, when the source output enable (SOE) signal is input. Here, the holding latch unit 130 is provided with n holding latches for storing n data. Also, the respective holding latches have sizes corresponding to the number of bits of the data. For example, the respective holding latches are set to have a size of k bits for storing the data having k bits.
The DAC 140 generates an analog signal corresponding to the bit value of the input digital data, and the DAC 140 selects any one of a plurality of gray scale voltages (or gray levels) corresponding to the bit value of the data supplied from the holding latch unit 130, thereby generating an analog data signal accordingly.
The amplifier 150 amplifies the digital data converted into the analog signal to a certain or predetermined level and thus outputs it through data lines on the panel.
As such, the data driver of FIG. 1 outputs one data per one horizontal period. That is, after the data driver samples and holds one digital R, G, B data (or one set of R, G, B data) during one horizontal period, it converts them into analog R, G, B data and amplifies and outputs them at a certain or predetermined width. In addition, when the holding latch unit 130 holds the R, G, B data corresponding to nth column line, the sampling latch unit 120 samples the R, G, B data corresponding to n+1th column line.
FIG. 2 is a block diagram showing the DAC shown in FIG. 1.
Referring to FIG. 2, a conventional DAC 140 includes: a reference voltage generator 142, a level shifter 144 and a switch array 146.
As shown in FIG. 2, the DAC 140 uses a reference voltage generator 142 having R-strings R1, R2, . . . Rn for generating correct gray scale voltages and/or gamma-corrections, and includes a ROM type of a switch array 146 for selecting the voltages generated through the reference voltage generator 142.
The DAC 140 includes a level shifter for converting and providing a voltage level for digital data input through the sampling latch unit (120 in FIG. 1) to the switch array 146.
The DAC 140 has a disadvantage because power consumption is increased due to a static current of the R-strings. In order to overcome this disadvantage, an approach has been developed in which the R-strings are designed with large resistance values for reducing the static current flowing in the R-strings, and in which the desired gray scale voltages are applied to the respective data lines by using an analog buffer in the respective channels as the amplifier 150. However, this approach has a disadvantage because image quality is deteriorated due to the output voltage difference between channels, when threshold voltages and mobility of certain transistors constituting portions of the analog buffer are not uniform.
Also, in implementing a gray scale of 6 bits, 6*64 switches for selecting one of 64 gray scale voltages (or gray levels) should be built in the respective channels, causing a disadvantage in that circuit area is greatly increased. In an embodiment of the prior art, the area of a DAC implementing the gray scale of 6 bits occupies more than half of the area of a data driver.
As the bits of a gray scale (or the number of gray levels) are increased, even more circuit area may be needed. For example, in implementing a gray scale of 8 bits, the circuit area of the DAC can be increase to more than four times the circuit area of the DAC implementing the gray scale of 6 bits.
Also, recently, a flat panel display device using a system on panel (SOP) process that uses polycrystalline silicon TFTs to integrate driver(s), etc., along with a display region on a substrate has been developed. The above described disadvantages of the conventional DAC, i.e., the problems of power consumption and/or area usage, and the problem of implementing the analog buffer as the amplifier, become even more pronounced, when the flat panel display device is implemented using the SOP process.